This invention relates to plant trays or plug sheets for growing seedlings in plug cups molded in the plant trays. Heretofore plants grown in the plug trays have not been supplied sufficient carbon dioxide and the lower foliage of the plants has become diseased and turned yellow. There has also been a buildup of heat on the surface area between the plug cups which has caused injury to the young plants. Another problem with the round cups used heretofore has been the growth of the roots in a spiraling configuration which has caused some of the plants to suffer from root strangulation.